This invention relates to hearing aids, and more particularly to an improvement to hearing aids.
A conventional hearing aid has not only the function of electrically amplifying an input sound to a microphone into a louder sound which is produced as an output from an earphone, but also the function of adjusting the output sound level to the hearing characteristic of a person having a hearing loss. Alternatively, a hearing aid may be designed so that its amplification is adjustable only over a small range, to provide a hearing aid with correct amplification for use by each person.
FIG. 1 is a block diagram showing the fundamental arrangement of a conventional hearing aid. An electrical signal from a microphone 1 is amplified by a preamplifier circuit 2. The output of the preamplifier circuit 2 is controlled by a variable resistor 3, the output of which is amplified by a main amplifier 4 to the extent that it can drive an ear-phone 5. Thus, the electrical signal from the microphone is converted into an acoustic signal. In some conventional hearing aids, the preamplifier circuit 2 has the function of varying the amplification band width according to a characteristic of the input signal from the microphone, so that the articulation index is maintained substantially unchanged when noise is present.
However, the conventional hearing aid suffers from difficulties when a loud microphone input sound occurs causing a loud output sound from the hearing aid. In this case, the microphone input sound is much louder than that which occurs when the hearing aid user hears conversational sounds or environmental sounds, thus causing maximum output sound. It goes without saying that the relation between the hearing characteristic of the hearing aid user and the maximum output sound obtained from the hearing aid is intentionally designed and controlled. If, when a high level microphone input sound occurs, such as a noise, the maximum output sound lasts for only an extremely short period of time, troubles are scarcely caused. However, when the maximum output sound lasts for several seconds to several tens of seconds, the energy of sound is accumulated in the user of the hearing aid, to cause the user to say "The hearing aid is noisy", with the result that he may refuse to use the hearing aid. The forcible use of a hearing aid is a great burden to the user. The above-described drawback may be eliminated by employing a hearing aid in which the maximum output sound is low so that no large burden is caused even when the maximum output sound lasts for a relatively long time. However, when such a hearing aid is used under ordinary conditions, the average sound level is excessively low, and therefore the reproduced sound is unsatisfactory or unclear. A method has been considered in which, when the output sound is noisy, a manual switch may be operated to decrease the maximum output sound level, thereby avoiding overloading. In general, a person having a hearing loss who wears a hearing aid which has been adjusted correctly merely gets sounds at the articulation index, in many cases, lower than that which the ordinary person gets. In the case where a person having a hearing loss intends to improve his low articulation index by enduring the overload, or where such a person intends to hear conversations under loud noises, often considers that he cannot hear clearly because of his hearing loss, even if the ordinary person cannot hear sufficiently in such a case. This further increases the load to the ears of the person with hearing loss. On the other hand, the sound which is heard when the surroundings are noisy, to the extent that the ordinary person cannot hear clearly, is the noise itself. Even under such a condition, the user of the hearing aid should hear conversations.